A pneumatic tire, in general, is formed in a tread unit thereof with a tread pattern constituted by various types of grooves, sipes and the like so that effective driving and braking performance as well as steering stability are ensured through increasing coefficient of friction between the tire and the road surface, or wet performance is improved through enhancing water drainage performance. As a typical tread pattern of this kind, conventionally, such a block pattern is well-known that a plurality of blocks are formed through partitioning by a plurality of main grooves which extend in the tire peripheral direction and a plurality of lug grooves which extend intersecting the main grooves (see Patent document 1).
FIG. 4, although it is not the one described in the patent document, is a plan view showing an example of a tread pattern in a development view of such a conventional pneumatic tire.
This pneumatic tire 100 has a tread pattern 110 which is symmetric with respect to a point on the equatorial plane CL of the tire, as shown in the drawing, in tread unit 101, a plurality of land rows 113 (five rows in the drawing) are formed by two main grooves 111 which are disposed sandwiching the tire equatorial plane CL and extending almost linearly in the tire peripheral direction, and two main grooves 112 disposed between each main groove 111 and the both tread ends TE and extending zigzag in the tire peripheral direction. In addition, a plurality of lug grooves 114 are provided, which are extending in inclined manner in the tire width direction intersecting the main grooves 111 and 112, so as to partition each land row 113 into a plurality of blocks 115 respectively by these lug grooves 114. Further, in this pneumatic tire 100, a plurality of sipes 116 are formed in each block 115 and in edge portions fronting the main groove 111, 112, or lug groove 114 of each block 115, one or two notches 117 are formed respectively.
Herein, the term notch stands for a notch which is cut into inside of the block from the edge of block, one end of which is opened out to each groove and the other end terminates inside of the block, and which is a recess that is formed locally at the edge of the block. In this conventional pneumatic tire 100, such notches 117 are formed in the block 115 to increase edge element of the block 115 that exerts edge effect, which enhances snow road performance and off-road performance through improving traction performance, braking performance, and the like of the pneumatic tire 100.
However, conventionally, it is usual that the notch 117 is formed such that the block 115 is cut from the surface thereof to the vicinity of the bottom of each groove 111, 112, and 114 linearly in the radial direction of the tire towards inside thereof so as to form substantially same depth as that of each groove 111, 112, and 114, or alternatively, the bottom is raised up so as to be formed slightly shallower by 1 or 2 mm from the bottom of each groove. Therefore in this conventional pneumatic tire 100, the rigidity of the block 115 is deteriorated and may cause to affect other tire performances.
That is to say, when the notches 117 are formed to have the same depth as that of the groove depth, off-road performance, snow road performance and the like can be improved, nevertheless the rigidity of blocks 115 (especially the blocks 115 in the shoulder land row 113 which is the outside-most in the direction of tire width) can not be ensured and cornering performance on dry surface, wet surface and the like is deteriorated, bringing about a tendency that steering stability such as handling and so on becomes poorer. On the other hand, when the notch 117 is formed to be shallower than the depth of the groove, the deterioration of rigidity of the block 115 and accompanied poorer cornering performance can be restrained to a certain extent, because of mass reduction of the notch 117, improvement effect of traction performance, braking performance and the like become smaller. In addition, although the notch 117 works effectively in improvement of traction performance and braking performance in snow road driving and so on, while ensuring the edge element mainly in back and forth direction (tire peripheral direction), there is a problem that it is not sufficient to obtain improvement effects of cornering performance (handling performance, anti-skid performance and the like) owing to increase of the edge element in transverse direction (tire width direction).
Consequently, in conventional pneumatic tires 100 provided with such notches 117, it is not feasible to coexist enhancement of snow road performance and off-road performance of tire through ensuring the traction performance and braking performance, and enhancement of the cornering performance on each road surface through ensuring rigidity and the like of the blocks 115, hence it is difficult to achieve effective improvement of the steering stability of tires.
Patent document 1: JP 05-229310 A